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Re: [Discuss-gnuradio] Tracking a Carrier Frequency

From: Bob McGwier
Subject: Re: [Discuss-gnuradio] Tracking a Carrier Frequency
Date: Thu, 06 Aug 2009 21:54:19 -0400
User-agent: Thunderbird (Windows/20090605)

The FFT divides the frequency range into bins. While you can do some computation based on the window function you use and how much power is in adjacent bins based on this knowledge and get a refined offset, from your proposal that sounds like it might be a step or two down the road for you rather than your first step. The gist of this is that the FFT will not be any more accurate than the bin width allows.

A better plan is to allow the FFT to "get you close" , and then use the FFT bin center as the initial condition for a PLL, and then use the actual phase locked loop based carrier routines with a noise bandwidth smaller than the bin width in the FFT. This will refine the estimate of the frequency nicely by using phase lock.

Don Latham wrote:
Hi Thomas: What you propose might work; it's a simple bang-bang servo
frequency control. It's success depends a lot on the frequency
characteristics of the drift you are trying to track. I have absolutely no
idea about the difficulty of implementation in Gnuradio, I'm sure that
depends a lot on the rest of your app.

Hello everyone,

I have an application in which I need the USRP to track a carrier
frequency being received. What I mean by "tracking the carrier" is I need
the USRP to stay tuned to the carrier frequency even if the carrier
frequency drifts (or if the USRP's frequency reference drifts).

I was thinking that I could peridically (perhaps once per second) take the
FFT, find the frequency of the greatest magnitude, and re-tune the USRP to
this frequency.

Is there any reason why this wouldn't work, and is there a better way? It
seems to me this would be a common problem, and there would already be a
good solution, but I haven't been able to find one.



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